The North Atlantic Oscillation—What Role for the Ocean? (original) (raw)
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Drivers of North Atlantic Oscillation Events
This work is set out to quantify the contribution of tropical and extratropical atmospheric forcing mechanisms to the formation of the North Atlantic Oscillation (NAO) pattern. Although the NAO varies on a wide range of time scales, we focus on 10Á60 d. At these time scales, mechanisms are at play in the atmosphere that can generate the characteristic dipole pattern. We focus on the tropical Rossby Wave Source (RWS) and extratropical eddy activity. Anomalous tropical and extratropical vorticity forcing associated with the NAO is derived from atmospheric reanalysis data and applied in an idealised barotropic model. Also, using winds from composites of the NAO, the vorticity forcing is derived inversely from the barotropic vorticity equation. Both types of forcing are imposed in the barotropic model in the tropics and extratropics, respectively. An important result is that the tropics dampen the NAO as a result of a negative feedback generated in the extratropics. The damping is strongest, about 30%, for the negative phase of the NAO. For the positive phase, the damping is about 50% smaller. The results show that the barotropic vorticity equation can represent the dynamics of both tropical and extratropical forcing related to the formation of the NAO patterns.
Geophysical Research Letters, 2014
The North Atlantic Oscillation (NAO) is a rapidly decorrelating process that strongly affects the climate over the Atlantic and the surrounding continents. Although the NAO itself is basically unpredictable on seasonal timescales using statistical methods, NAO forcing is here shown to significantly affect sea surface temperatures (SSTs) evolving on those timescales. Results using linear inverse modeling (LIM) imply that the NAO index and its convolution with deterministic SST dynamics account for nearly half the unpredictable component of north tropical Atlantic SST at lead times greater than 9 months; adding this component to hindcasts at a lead of 48 weeks increases correlation with north tropical Atlantic SST from about 0.4 to about 0.6. Rapid fluctuations during boreal winter and spring, when the NAO is strongest, affect SST predictability throughout the entire year.
A Study of the Interaction of the North Atlantic Oscillation with Ocean Circulation
Journal of Climate, 2001
Observed patterns of wind stress curl and air-sea heat flux associated with the North Atlantic oscillation (NAO) are used to discuss the response of ocean gyres and thermohaline circulation to NAO forcing and their possible feedback on the NAO. The observations motivate, and are interpreted in the framework of, a simple mathematical model that couples Ekman layers, ocean gyres, and thermohaline circulation to the atmospheric jet stream. Meridional shifts in the zero wind stress curl line are invoked to drive anomalies in ocean gyres, and north-south dipoles in air-sea flux drive anomalous thermohaline circulation. Both gyres and thermohaline circulation play a role in modulating sea surface temperature anomalies and hence, through air-sea interaction, the overlying jet stream. The model, which can be expressed in the form of a delayed oscillator with ocean gyres and/or thermohaline circulation providing the delay, identifies key nondimensional parameters that control whether the ocean responds passively to NAO forcing or actively couples. It suggests that both thermohaline circulation and ocean gyres can play a role in coupled interactions on decadal timescales. 1 The NAO anomaly fields discussed here were computed by regressing NCEP-NCAR reanalysis fields onto the wintermean (DJF) NAO index of . They correspond to a (Hurrell) NAO index of ϩ1 (See Visbeck et al. 1998).
The Relation between the North Atlantic Oscillation and SSTs in the North Atlantic Basin
Journal of Climate, 2004
The authors use the notion of Granger causality to investigate the relationship between the North Atlantic Oscillation (NAO) index and the sea surface temperatures (SSTs) over the Northern Hemisphere. The Granger causality analysis ensures that any apparent oceanic influence upon the atmosphere (as measured by the NAO) is provided by the ocean and is not related to preexisting conditions within the NAO itself (and vice versa when looking at the atmospheric influence upon the ocean). Although this statistical technique does not imply physical forcing of one field on the other, it is generally more reliable compared to the simple lead/lagged correlation. Using this technique, the authors find that on seasonal time scales, the preceding NAO anomalies' influence on the wintertime SST field is rather restricted. Conversely, preceding SST anomalies have a statistically significant causal effect on the wintertime NAO. However, the causal relation between preceding SSTs and the winterti...
The role of Atlantic Ocean-atmosphere coupling in affecting North Atlantic oscillation variability
We review the role of ocean-atmosphere interactions over the Atlantic sector in North Atlantic Oscillation (NAO) variability. The emphasis is on physical mechanisms, which are illustrated in simple models and analyzed in observations and numerical models. Some directions of research are proposed to better assess the relevance of Atlantic air-sea interactions to observed and simulated NAO variability. Figure 4. Intraseasonal (right thick curve) and interannual (left thick curve) spectrum of an observed NAO index. The red noise or AR(1) spectrum (lowest thin line), and its a priori (middle thin line) and a posteriori (upper thin line) 95% confidence levels are also shown. From Feldstein [2000; his Figure 4].
A Hemispheric Mechanism for the Atlantic Multidecadal Oscillation
Journal of Climate, 2007
The physical processes associated with the ∼70-yr period climate mode, known as the Atlantic multidecadal oscillation (AMO), are examined. Based on analyses of observational data, a deterministic mechanism relying on atmosphere–ocean–sea ice interactions is proposed for the AMO. Variations in the thermohaline circulation are reflected as uniform sea surface temperature anomalies in the North Atlantic. These anomalies are associated with a hemispheric wavenumber-1 sea level pressure (SLP) structure in the atmosphere that is amplified through atmosphere–ocean interactions in the North Pacific. The SLP pattern and its associated wind field affect the sea ice export through Fram Strait, the freshwater balance in the northern North Atlantic, and consequently the strength of the large-scale ocean circulation. It generates sea surface temperature anomalies with opposite signs in the North Atlantic and completes a negative feedback. The authors find that the time scale of the cycle is assoc...
A Damped Decadal Oscillation in the North Atlantic Climate System
Journal of Climate, 2003
A simple stochastic atmosphere model is coupled to a realistic model of the North Atlantic Ocean. A northsouth SST dipole, with its zero line centered along the subpolar front, influences the atmosphere model, which in turn forces the ocean model by surface fluxes related to the North Atlantic Oscillation. The coupled system exhibits a damped decadal oscillation associated with the adjustment through the ocean model to the changing surface forcing. The oscillation consists of a fast wind-driven, positive feedback of the ocean and a delayed negative feedback orchestrated by overturning circulation anomalies. The positive feedback turns out to be necessary to distinguish the coupled oscillation from that in a model without any influence from the ocean to the atmosphere. Using a novel diagnosing technique, it is possible to rule out the importance of baroclinic wave processes for determining the period of the oscillation, and to show the important role played by anomalous geostrophic advection in sustaining the oscillation.
Geophysical Research Letters, 2000
The North Atlantic Oscillation (NAO) exhibits variations at interannual to multidecadal time scales and is associated with climate variations over eastern North America, the North Atlantic, Europe, and North Africa. Therefore, it is very important to understand causes of these NAO variations and assess their predictability. It has been hypothesized, based on observations, that sea surface temperature (SST) and sea-ice variations in the North Atlantic Ocean influence the NAO. We describe results of an ensemble of sixteen experiments with an atmospheric general circulation model in which we used observed SST and sea-ice boundary conditions globally during 1949-1993. We show that multiyear NAO and associated climate variations can be simulated reasonably accurately if results from a large number of experiments are averaged. We also show that the ambiguous results of previous NAO modeling studies were strongly influenced by the ensemble size, which was much smaller than that in the present study. The implications of these results for understanding and predictability of the NAO are discussed.
A Simple Model of the Response of the Atlantic to the North Atlantic Oscillation
Journal of Climate, 2014
The response of an idealized Atlantic Ocean to wind and thermohaline forcing associated with the North Atlantic Oscillation (NAO) is investigated both analytically and numerically in the framework of a reduced-gravity model. The NAO-related wind forcing is found to drive a time-dependent “leaky” gyre circulation that integrates basinwide stochastic wind Ekman pumping and initiates low-frequency variability along the western boundary. This is subsequently communicated, together with the stochastic variability induced by thermohaline forcing at high latitudes, to the remainder of the Atlantic via boundary and Rossby waves. At low frequencies, the basinwide ocean heat content changes owing to NAO wind forcing and thermohaline forcing are found to oppose each other. The model further suggests that the recently reported opposing changes of the meridional overturning circulation in the Atlantic subtropical and subpolar gyres between 1950–70 and 1980–2000 may be a generic feature caused by...